ia32: Factor out code to determine the frame offset for an x86_addr_t.
ia32: Factor out code to determine the frame offset for an x86_addr_t.

MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm